The invention relates to an electrophotographic copying process, and more particularly, to an electrophotographic copying process utilizing a photosensitive member which includes a conductive layer carrying a sequential lamination of a photoconductive layer which is sensitive to visible light (hereinafter referred to as a visible light photoconductive layer) and an insulator or a photoconductor which is different from the visible light photoconductive layer and serves a function independent thereof (hereinafter referred to as a function layer), forming an electrostatic latent image of an original to be copied by trapping charge on opposite sides of either of the visible light photoconductive layer and the function layer in such a condition that the layer which is not retaining the charge is one of photoconductive layers forming the photosensitive member, and repeatedly employing the electrostatic latent image to produce a desired number of copies.
A variety of electrophotographic copying apparatus have been hitherto proposed in the prior art which produces a plurality of copies by repeatedly employing an electrostatic latent image of originals which has been formed on the photosensitive member. By way of examples, there have been disclosed techniques such as (I) a procedure for forming the same copy images on a plurality of copy sheets by repeatedly subjecting an electrophotographic latent image which has been formed on a photosensitive member only to a developing step with toner and a transfer step and (II) a procedure for producing a plurality of copy images by initially forming a secondary latent image a plurality of times by repeating a step for forming the secondary latent image on a dielectric recording medium by modulating a corona ion current in accordance with an electrostatic latent image which has been formed on a screen-shaped photosensitive member, followed by a step for developing the secondary latent image, and subsequently followed by repeating a step for transferring the developed toner image onto a record sheet. In such an electrophotographic copying process that a plurality of copies are produced from the same latent image, it is necessary that in order to obtain a plurality of copy images of good quality, the electrostatic latent image which has been formed on the photosensitive member is maintained in a stable manner over a number of steps for copying. However, it is practical that there arises 1 a phenomenon in which charge on a member for retaining the latent image decays with the lapse of time, namely, a dark decay of the photosensitive member and a leakage of charge on an insulator, and 2 an injection of charge from the outside and a migration of charge to the outside. For example, the latent image may deteriorate by reason such as a charge migration through a developer and a charge injection caused during a transfer step in the electrophotographic copying process of example (I) described above, and a charge injection onto the screen-shaped photosensitive member caused by a detouring of a corona ion current which may arise during a formation of the secondary latent image in the electrophotographic copying process of example (II) described above. For the purpose of preventing such deterioration of the electrostatic latent image with the lapse of time, improvements in the electrophotographic copying process have been proposed. Specifically, one of such improvements is an electrophotographic copying process which employs a photosensitive member which includes a conductive layer carrying a sequential lamination of a visible light photoconductive layer and an insulator layer and forms an electrostatic latent image by trapping charges of opposite polarities on opposite sides of the insulator layer, respectively. Another one of such improvements is an electrophotographic copying process for producing a plurality of copies which employs a photosensitive member similar to that in the above described process, which member replaces the insulator layer with a photoconductive layer which is sensitive only to ultraviolet rays and forms an electrostatic latent image by trapping charge on opposite sides of the photoconductive layer which is sensitive to ultraviolet rays. Also, a further electrophotographic copying process for producing a plurality of copies has been proposed which prevents a migration of charge owing to a direct contact with a developer by trapping charge on opposite sides of a visible light photoconductive layer or an inner layer of a compound layer photosensitive member to form an electrostatic latent image. However, a conventional electrophotographic copying process for producing a plurality of copies which employs such a compound layer photosensitive member is favorably applicable in view of a fact that a decay of the latent image with the lapse of time or a charge migration through a developer during a developing step can be prevented. It should be understood, however, that such processes do not solve the problem of deterioration of the latent image by a charge injection which may be caused during a transfer step in the process (I) described above, or by a charge injection by a detouring of a corona ion current which may be caused during the step for forming a secondary latent image in the process (II) described above.
In the following, the electrophotographic copying processes for producing a plurality of copies in the above-mentioned examples (I) and (II) which employ such a compound layer photosensitive member will be described.
FIGS. 1(A) through (E) schematically illustrate a sequence of copying steps for the electrophotographic copying process (I) aforementioned. A photosensitive member 1 includes a conductive layer 2 on which is successively laminated a visible light photoconductive layer 3 and a light transmitting insulator layer 4. As indicated in FIG. 1(A), a uniform charging with a positive polarity, for example, is performed with a corona charger 5 in darkness. The result is such that when the dark resistance of visible light photoconductive layer 3 is low, charges of opposite polarities are trapped on opposite sides of insulator layer 4, as indicated in the figure. When the dark resistance of photoconductive layer 3 is sufficiently high, the charges are trapped on opposite sides of insulator layer 4 by a uniform charging simultaneously with a uniform exposure by visible light. Subsequently, as indicated in FIG. 1(B), a neutralization is performed by a charging with the opposite polarity or with an alternating current with a corona charger 6 simultaneously with an irradiation of an optical image 12 by visible light so that a surface potential of photosensitive member 1 reaches substantially zero volts. Thereafter, a uniform exposure by visible light 13 is applied, as indicated in FIG. 1(C). As a consequence, in a dark area of the image, a positive charge is trapped on the exposed surface of insulator layer 4 and a negative charge is trapped between insulator layer 4 and visible light photoconductive layer 3, respectively, thus forming an electrostatic latent image. The latent image, in a step for developing indicated in FIG. 1(D), is visualized by a negatively charged toner 8 under an operation of a developing unit 7 comprising a magnet roller, for example. Subsequently, in a step for transferring indicated in FIG. 1(E), the toner image on photosensitive member 1 is transferred onto a copy sheet 11 under an operation of a bias transfer roller 10 connected to a transfer bias source 9, for example. Thereafter, a plurality of copies can be produced with the latent image which has been formed on photosensitive member 1 by repeating a sequence of only those steps for developing and for transferring indicated in FIGS. 1(D) and (E).
Since the electrophotographic copying process for producing a plurality of copies indicated in FIGS. 1(A) through (E) forms the latent image by trapping charges of opposite polarities on opposite sides of insulator layer 4, respectively, a decay of the latent image with the lapse of time can be prevented. However, charge which is trapped on the surface of insulator layer 4 during the step for developing indicated in FIG. 1(D) migrates slightly through a developer, or a small quantity of charge is injected onto a non-image area (a bright area of the image) of photosensitive member 1 in accordance with the transfer electric field during the step for transferring indicated in FIG. 1(E) and a quantity of charge which issues from photosensitive member 1 and is injected thereonto is increased by a repetition of the steps for developing and for transferring, resulting in a decay of the latent image. With such decay of the latent image, as indicated in FIG. 2 with regard to the reflection factor of an original or the relation between exposures and surface potentials, a difference between potentials on the bright and the dark area of the image is sufficiently high, as shown with a solid line a.sub.0, while after the repetition of the steps for developing and for transferring a potential on the dark area falls and a potential on the bright area rises, resulting in a decrease of the potential difference thereon, as shown with a broken line b.sub.0. Consequently, the maximum density of resulted copies decreases as the number of copies produced increases and photographic fogging is increased, thus producing a low contrast copy image.
FIGS. 3(A) through (D) schematically illustrate a sequence of copying steps for an electrophotographic copying process in accordance with the process (II) in the foregoing. As shown, a screen-shaped photosensitive member 21 is formed in such a manner that a conductive mesh 22 is covered with a visible light photoconductor 23 therearound so as to be partially exposed and further an insulator 24 is laminated on the visible light photoconductor 23. As indicated in FIG. 3(A), a corona charging with positive polarity, for example, is uniformly applied from the insulator 24 side simultaneously with a uniform exposure 15 by light rays on the screen-shaped photosensitive member 21 so that charges of opposite polarities may be trapped on opposite sides of insulator 24 in the same manner as shown in FIG. 1(A). Subsequently, as indicated in FIG. 3(B), a corona charging 16 with the polarity opposite to that in FIG. 3(A) is performed simultaneously with an irradiation of an optical image of an original 25 from the insulator 24 side so that charge with a polarity opposite to that in a dark area of the image D may be trapped on opposite sides of insulator 24 in a bright area of the image L. Thereafter, as indicated in FIG. 3(C), a uniform exposure 17 is applied to the photosensitive member 21 to form an electrostatic latent image wherein a positive charge is trapped on the surface of insulator 24 in the bright area and a negative charge is trapped on the surface of insulator 24 in the dark area, respectively. After the latent image has been formed on the photosensitive member 21 as described above, a secondary latent image is formed on an insulator layer 27, as indicated in FIG. 3(D), which is a surface layer of a dielectric recording medium 28 which includes insulator layer 27 and a conductor 26. Specifically, a corona discharge wire 29 is disposed on the conductive mesh 22 side of photosensitive member 21 and dielectric recording medium 28 is disposed on the insulator 24 side. Further, a corona discharge power source 30a and an acceleration power source 30b are connected between corona discharge wire 29 and conductive mesh 22 and between conductive mesh 22 and conductor 26 of dielectric recording medium 28, respectively. To such arrangement thus formed is applied a voltage so as to develop a potential difference in the direction from corona discharge wire 29 through photosensitive member 21 to dielectric recording medium 28 and a corona ion current with negative polarity is projected toward dielectric recording medium 28 from corona discharge wire 29, as shown with broken lines. At this time, in the bright area of photosensitive member 21, since an electric field .alpha. is developed at an opening thereof in the direction that passing of the corona ion current with negative polarity is obstructed, the corona ion current flows into conductive mesh 22 which is exposed. In contrast, in the dark area of the image, since an electric field .beta. is developed at an opening thereof in the direction that passing of the corona ion current with negative polarity is promoted, the corona ion current reaches dielectric recording medium 28 through the opening. As a result, a secondary latent image is formed on dielectric recording medium 28, which corresponds to an electrostatic latent image formed on photosensitive member 21. After the secondary latent image formed on dielectric recording medium 28 is developed with toner, a copied image can be produced by transferring the toner image onto a copy sheet. Additionally, it is to be understood that after the toner image has been transferred, the secondary latent image is formed by repeating steps for cleaning and neutralizing dielectric recording medium 28, as shown in FIG. 3(D), whereby a plurality of copies can be produced with the electrostatic latent image which has been formed on screen-shaped photosensitive member 21.
In the electrophotographic copying process shown in FIG. 3, charge is trapped on opposite sides of insulator 24, in the same manner as in FIG. 1, to form the electrostatic latent image. Consequently, the latent image has little decay but is deteriorated by an injection of external charge caused by a detouring of the corona ion current into insulator 24 of photosensitive member 21 during a step for producing a plurality of copies or a step for forming the secondary latent image, a secondary emission on the surface of dielectric recording medium 28 and the like.